[clang] [llvm] [clang][HLSL][SPRI-V] Add convergence intrinsics (PR #80680)

[Nathan Gauër via cfe-commits]

Keenuts wrote:

@arsenm would you be fine with those codegen changes as-is? Given that the convergent/no-convergent switch will be done later, depending on when the required IR change is merged?

https://github.com/llvm/llvm-project/pull/80680